Counter-rotating low pressure turbine with gear system mounted to mid turbine frame

ABSTRACT

A gas turbine engine includes a shaft defining an axis of rotation. An outer turbine rotor directly drives the shaft and includes an outer set of blades. An inner turbine rotor has an inner set of blades interspersed with the outer set of blades. The inner turbine rotor is configured to rotate in an opposite direction about the axis of rotation from the outer turbine rotor. A gear system couples the inner turbine rotor to the shaft and is configured to rotate the inner set of blades at a faster speed than the outer set of blades. The gear system is mounted to a mid-turbine frame.

BACKGROUND

A typical jet engine has multiple shafts or spools that transmit torquebetween turbine and compressor sections of the engine. In one example, alow speed spool generally includes a low shaft that interconnects a fan,a low pressure compressor, and a low pressure turbine. In order toachieve a desirable high pressure core ratio, a long low shaft isrequired. In contrast, to increase an engine's power density, there is acountering goal of shortening the overall engine length. Thus,historically these two concepts have been at odds.

SUMMARY

In one exemplary embodiment, a gas turbine engine includes a shaftdefining an axis of rotation, a turbine section, and a gear system. Theturbine section includes: (a) an outer turbine rotor that directlydrives the shaft and that includes a first set of blades; and (b) aninner turbine rotor that has a second set of blades interspersed withthe first set of blades. The inner turbine rotor is configured to rotatein an opposite direction about the axis of rotation from the outerturbine rotor. The gear system couples the inner turbine rotor to theshaft and is configured to rotate the second set of blades at a fasterspeed than the first set of blades. The gear system is mounted to amid-turbine frame.

In a further embodiment of the above, the gear system is mounted to amid-turbine frame by a bearing.

In a further embodiment of any of the above, the gear system includes asun gear engaged to the inner rotor, a plurality of star gears inmeshing engagement with the sun gear, and a ring gear in meshingengagement with the star gears.

In a further embodiment of any of the above, the sun gear is fixed forrotation with an aft end of the inner turbine rotor.

In a further embodiment of any of the above, the star gears aresupported within a carrier that is fixed to the mid-turbine frame.

In a further embodiment of any of the above, a fore end of the outerturbine rotor is coupled to the ring gear.

In a further embodiment of any of the above, a first bearing supports afore end of the inner turbine rotor for rotation relative to the shaftand a second bearing supports an aft end of the inner turbine rotor forrotation relative to the shaft.

In a further embodiment of any of the above, a third bearing supportsthe shaft for rotation relative to the mid-turbine frame.

In a further embodiment of any of the above, the first, second, andthird bearings are axially spaced apart from each other parallel to theaxis of rotation with the third bearing being located forward of thefirst bearing.

In a further embodiment of any of the above, a low pressure turbinestatic case has an aft end unsupported and a fore end connected to amid-turbine frame outer case.

In a further embodiment of any of the above, a fore end of the shaft isassociated with a counter-rotating low pressure compressor.

In another exemplary embodiment, a gas turbine engine comprises a coreair flowpath and a shaft supporting a compressor section and a turbinesection arranged within the core flow path. The turbine section includesa counter-rotating low pressure turbine comprising an outer rotordirectly driving the shaft and having a first set of blades and an innerrotor having a second set of blades interspersed with the first set ofblades. The inner rotor is configured to rotate in an opposite directionabout the axis of rotation from the first rotor. A gear system couplesthe inner rotor to the shaft and is configured to rotate the second setof blades at a faster speed than the first set of blades. The gearsystem is supported by a mid-turbine frame.

In a further embodiment of any of the above, the compression sectionincludes a high pressure compression section that has a pressure ratioof approximately 23:1.

In a further embodiment of any of the above, the compressor sectionincludes a counter-rotating low pressure compressor driven by the shaft.

In a further embodiment of any of the above, the gear system includes asun gear engaged to the second rotor, a plurality of star gears inmeshing engagement with the sun gear, and a ring gear in meshingengagement with the star gears.

In a further embodiment of any of the above, the sun gear is fixed forrotation with a fore end of the inner rotor, the star gears aresupported within a carrier that is fixed to the mid-turbine frame, and afore end of the outer rotor is coupled to the ring gear.

In a further embodiment of any of the above, a first bearing supports afore end of the inner rotor for rotation relative to the shaft, a secondbearing supports an aft end of the inner rotor for rotation relative tothe shaft, and a third bearing supports the shaft for rotation relativeto the mid-turbine frame.

In a further embodiment of any of the above, the first, second, andthird bearings are axially spaced apart from each other parallel to theaxis of rotation with the third bearing being located forward of thefirst bearing.

In a further embodiment of any of the above, the first and thirdbearings are roller bearings and the second bearing is a ball bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be further understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 schematically illustrates a gas turbine engine embodiment.

FIG. 2 is a cross-sectional view of an engine upper half showing anexample of a non-counter-rotating configuration and an engine lower halfshowing an embodiment of a counter-rotating low pressure compressorarchitecture and counter-rotating low pressure turbine architecture of agas turbine engine.

FIG. 3 shows an enlarged view of the low pressure compressor shown inFIG. 2.

FIG. 4 shows an enlarged view of the low pressure turbine shown in FIG.2.

FIG. 5 shows a schematic view of the lower pressure compressor shown inFIG. 2.

FIG. 6 shows a schematic view of the lower pressure turbine shown inFIG. 2.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. Alternative engines mightinclude an augmentor section (not shown) among other systems orfeatures. The fan section 22 drives air along a bypass flowpath B whilethe compressor section 24 drives air along a core flowpath C forcompression and communication into the combustor section 26 thenexpansion through the turbine section 28. Although depicted as aturbofan gas turbine engine in the disclosed non-limiting embodiment, itshould be understood that the concepts described herein are not limitedto use with turbofans as the teachings may be applied to other types ofturbine engines including three-spool architectures.

The engine 20 generally includes a low speed spool 30 and a high speedspool 32 mounted for rotation about an engine central longitudinal axisA relative to an engine static structure 36 via several bearing systems38. It should be understood that various bearing systems 38 at variouslocations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects a fan 42, a low pressure (or first) compressor section 44and a low pressure (or first) turbine section 46. The inner shaft 40 isconnected to the fan 42 through a geared architecture 48 to drive thefan 42 at a lower speed than the low speed spool 30. The high speedspool 32 includes an outer shaft 50 that interconnects a high pressure(or second) compressor section 52 and high pressure (or second) turbinesection 54. A combustor 56 is arranged between the high pressurecompressor 52 and the high pressure turbine 54. A mid-turbine frame 57of the engine static structure 36 is arranged generally between the highpressure turbine 54 and the low pressure turbine 46. The mid-turbineframe 57 supports one or more bearing systems 38 in the turbine section28. The inner shaft 40 and the outer shaft 50 are concentric and rotatevia bearing systems 38 about the engine central longitudinal axis A,which is collinear with their longitudinal axes. As used herein, a “highpressure” compressor or turbine experiences a higher pressure than acorresponding “low pressure” compressor or turbine.

The core airflow C is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 57 includes airfoils 59 whichare in the core airflow path. The turbines 46, 54 rotationally drive therespective low speed spool 30 and high speed spool 32 in response to theexpansion.

The engine 20 in one example is a high-bypass geared aircraft engine. Ina further example, the engine 20 bypass ratio is greater than about six(6), with an example embodiment being greater than ten (10), the gearedarchitecture 48 is an epicyclic gear train, such as a star gear systemor other gear system, with a gear reduction ratio of greater than about2.3 and the low pressure turbine 46 has a pressure ratio that is greaterthan about 5. In one disclosed embodiment, the engine 20 bypass ratio isgreater than about ten (10:1), the fan diameter is significantly largerthan that of the low pressure compressor 44, and the low pressureturbine 46 has a pressure ratio that is greater than about 5:1. Lowpressure turbine 46 pressure ratio is pressure measured prior to inletof low pressure turbine 46 as related to the pressure at the outlet ofthe low pressure turbine 46 prior to an exhaust nozzle. It should beunderstood, however, that the above parameters are only exemplary of oneembodiment of a geared architecture engine and that the presentinvention is applicable to other gas turbine engines including directdrive turbofans.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the engine 20 is designedfor a particular flight condition—typically cruise at about 0.8 Mach andabout 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft, withthe engine at its best fuel consumption—also known as “bucket cruiseThrust Specific Fuel Consumption (‘TSFCT’)”—is the industry standardparameter of lbm of fuel being burned per hour divided by lbf of thrustthe engine produces at that minimum point. “Fan pressure ratio” is thepressure ratio across the fan blade alone, without a Fan Exit Guide Vane(“FEGV”) system. The low fan pressure ratio as disclosed hereinaccording to one non-limiting embodiment is less than about 1.45. “Lowcorrected fan tip speed” is the actual fan tip speed in ft/sec dividedby an industry standard temperature correction of [(Tambient degR)/518.7)^0.5]. The “Low corrected fan tip speed” as disclosed hereinaccording to one non-limiting embodiment is less than about 1150ft/second.

Referring to FIGS. 2 and 3, a geared turbofan architecture with acounter-rotating low pressure compressor (LPC) 60 and counter-rotatinglow pressure turbine (LPT) 62 is provided, which significantly reduces alength of the low speed or inner shaft 40 as compared to anon-counter-rotating configuration, an example of which is shown in FIG.1 and in the upper half of FIG. 2. This non-rotating configuration inthe upper half of FIG. 2 is included for the purposes of a lengthcomparison to the counter-rotating LPC and counter-rotating LPTconfigurations shown in the lower half of FIG. 2. The engine has a highpressure core, schematically indicated at 64. It is to be understoodthat the high pressure core 64 includes the combustor 56 and the highspool 32 (i.e., the high pressure compressor 52, the high pressureturbine 54, and the high shaft 50) shown in FIG. 1. The high pressurecompressor 52 has a high pressure core ratio of 23:1, for example. Toretain this ratio, as well as providing a desired low shaft diameter andspeed, a combination of the counter-rotating LPC 60 and LPT 62 isutilized as shown in the lower half of FIG. 2.

One example of the LPC 60 is found in U.S. Pat. No. 7,950,220, which isassigned to the same assignee as the subject invention, and which ishereby incorporated by reference. In this example, which is shown inFIG. 2, the LPC 60 includes a counter-rotating compressor hub 70 withblade stages 72, 74, and 76 interspersed with blade stages 78 and 80 ofthe low speed spool 30. The counter-rotating compressor hub 70 may bedriven by a transmission 82. The transmission 82 is also schematicallyillustrated in FIG. 5. In one example, the transmission 82 is anepicyclic transmission having a sun gear 84 mounted to the low shaft 40.A circumferential array of externally-toothed star gears 86 are inmeshing engagement with the sun gear 84. The star gears 86 are carriedon journals 88 carried by a carrier 90. The carrier 90 is fixedlymounted relative to an engine static structure 92. The static structure92 is coupled to the low shaft 40 via multiple bearing systems 94 and 96to permit rotation of the low shaft 40.

The transmission 82 further includes an internally-toothed ring gear 98encircling and in meshing engagement with the star gears 86. The ringgear 98 is supported relative to the static structure 92 by one or morebearing systems 100 and 102. The transmission 82 causes acounter-rotation of ring gear 98. As the compressor hub 70 is engagedwith the ring gear 98, the transmission 82 causes a counter-rotation ofthe compressor hub 70 (and blades 72, 74, 76) relative to the low speedspool 30. Fan blades 104 of the fan section 22 are mounted via a hub 106to the low shaft 40. In addition, and low pressure compressor blades 78,80 are also mounted to the hub 106 via a blade platform ring 108. As aresult of the foregoing, the fan blades 104 and the low pressurecompressor blades 78, 80 co-rotate with the low shaft 40.

An outboard surface of the platform ring 108 locally forms an inboardboundary of a core flowpath 110. The blades of stages 78 and 80 extendfrom inboard ends fixed to the platform ring 108 to free outboard tips.In the example shown, the blades of the downstreammost stage 76 of thehub 70 are mounted to an outboard end of a support 112. The outboardends of the blades of the stage 76 are secured relative to a shroud ring114. An inboard surface of the shroud ring 114 forms a local outboardboundary of the core flowpath 110. The outboard ends of the blades ofthe stages 72 and 74 are mounted to the shroud ring 114. The support 112is affixed to the ring gear 98 to drive rotation of the blades of stage76 and, through the shroud ring 114, the blades of stages 72 and 74.

As shown in the upper half of FIG. 2, in one typicalnon-counter-rotating configuration, the engine 20 without acounter-rotating compressor or turbine has an overall length L1 definedfrom a foremost surface of the fan blade 104 to an aftmost end of aturbine exhaust case 118. The LPC configuration 60 provides a lengthreduction L2 by utilizing a counter-rotating compressor architecture.The LPT configuration 62 provides another length reduction L3 byutilizing a counter-rotating turbine architecture. One example of a LPTis found in United States Publication No. 2009/0191045 A1, which isassigned to the same assignee as the subject invention, and which ishereby incorporated by reference.

FIGS. 2 and 4 show another example of a LPT 62 having a counter-rotatingconfiguration with a gear system 116 mounted to the mid turbine frame134. The gear system 116 is also schematically illustrated in FIG. 6. Asa result, no turbine exhaust case 118 is needed, which furthercontributes to the overall amount of length reduction L3 by shorteningthe LPT static case portion. In this example, the LPT 62 has an innerset of blades 120 that are coupled to the low shaft 40 via the gearsystem 116 and an outer set of blades 122 interspersed with the innerset of blades 120. In one example, the number of stages in the inner setof blades 120 is equal to the number of stages in the outer set ofblades 122. The outer set of blades 122 is directly coupled to the shaft40. The outer blades 122 rotate in an opposite direction about the axisof rotation from the inner set of blades 120.

The outer set of blades 122 is fixed to an outer rotor 126 that directlydrives the low shaft 40, i.e. the low shaft 40 and outer set of blades122 rotate at a common speed. The inner set of blades 120 is fixed to aninner rotor 124 that drives the gear system 116. Bearings 130, 132rotatably support the inner rotor 124. Bearing 130 supports an aft endof the inner rotor 124 for rotation relative to the low shaft 40, andbearing 132 supports a fore end of the inner rotor 124 for rotationrelative to the shaft 40. In one example, the aft bearing 130 is a ballbearing and the fore bearing 132 is a roller bearing. A bearing 146supports the low shaft 140 for rotation relative to the mid-turbineframe 134. In one example configuration, the shaft bearing 146 and thefore and aft bearings 132, 130 for the inner rotor 126 are axiallyspaced apart from each other parallel to the axis A. The shaft bearing146 is located forward of the fore bearing 132. In one example, bothbearings 132, 146 are roller bearings.

A mid-turbine frame 134 comprises a static structure that extends to anouter case portion 136. The outer case portion 136 is attached to a foreend of a LPT static case 138, which surrounds the inner 120 and outer122 sets of blades. An aft end of the LPT static case 138 is unsupportedsince there is no turbine exhaust case 118.

The gear system 116 includes a sun gear 140 that is fixed for rotationwith a fore end of the inner rotor 124. A circumferential array ofexternally-toothed star gears 142 are in meshing engagement with the sungear 140. The star gears 142 are supported by a carrier 144 that isfixed to the mid-turbine frame 134.

A ring gear 148 is in meshing engagement with the star gears 142 whichare driven by the sun gear 140. The fore end of the inner rotor 124drives the sun gear 140. In the example shown in FIG. 2, the fore end ofthe outer rotor 126 is configured to be driven by the ring gear 148. Thefore end of the outer rotor 126 is supported relative to the mid-turbineframe 134 by a bearing 150. Thus, the inner set of blades 120 is drivenat a faster speed than the outer set of blades 122. In one example, thegear system has a ratio within a range of between about 0.5:1 and about5.0:1.

In this configuration, the gear system 116 is upstream or forward of theLPT 62. Specifically, the gear system 116 is positioned forward of theinterspersed blades 120, 122 and is surrounded by the mid-turbine frame.The carrier 144 for the star gears 142 is fixed to the mid-turbine frame134. This counter-rotating configuration allows the overall length ofthe LPT static case 138 to be shortened compared to anon-counter-rotating configuration, and eliminates the need for aturbine exhaust case 118. This results in a weight reduction as well ascontributing to the desired length reduction L3.

The low shaft 40 receives a portion of the overall driving inputdirectly from the outer set of blades 122 and a remaining portion of theoverall driving input is provided by the inner set of blades 120 via thegear system 116. The outer set of blades 122 is configured to rotate ata lower speed and in an opposite direction from the inner set of blades120. Spinning the inner set of blades 120 at a higher speed takesadvantage of the existing turbine disks' ability to handle higherspeeds. This configuration provides a geared turbofan architecture witha long, slow turning low shaft 40, which enables the use of a highpressure ratio core. Further, this configuration provides forsignificant length reduction as compared to prior configurations.

It should be understood that the LPC 60 described above is just oneexample configuration, and that the LPT 62 described above could beutilized with various other LPC configurations. Further, the LPT 62could also be used in a configuration that does not include acounter-rotating LPC.

As a result of the foregoing improvements, an engine has been inventedthat includes both a desirable high pressure core ratio, while at thesame time reducing the overall engine length, thereby maximizing theengine's power density.

Although an example embodiment has been disclosed, a worker of ordinaryskill in this art would recognize that certain modifications would comewithin the scope of the claims. For that reason, the following claimsshould be studied to determine their true scope and content.

What is claimed is:
 1. A gas turbine engine comprising: a shaft definingan axis of rotation; a turbine section including a high pressure turbinesection and a low pressure turbine section, comprising: an outer turbinerotor directly driving the shaft, the outer rotor including a first setof blades; and an inner turbine rotor having a second set of bladesinterspersed with the first set of blades, the inner turbine rotorconfigured to rotate in an opposite direction about the axis of rotationfrom the outer turbine rotor; a gear system coupling the inner turbinerotor to the shaft and configured to rotate the second set of blades ata faster speed than the first set of blades, the gear system mounted toa mid-turbine frame that is disposed between the high pressure turbinesection and the low pressure turbine section; a first bearing supportinga fore end of the inner turbine rotor for rotation relative to theshaft; a second bearing supporting an aft end of the inner turbine rotorfor rotation relative to the shaft; and a third bearing supporting theshaft for rotation relative to the mid-turbine frame.
 2. The gas turbineengine according to claim 1, wherein the first, second, and thirdbearings are axially spaced apart from each other parallel to the axisof rotation with the third bearing being located forward of the firstbearing.
 3. A gas turbine engine comprising: a core air flowpath; ashaft supporting a compressor section and a turbine section arrangedwithin the core flowpath, the turbine section includes a high pressureturbine section and a low pressure turbine section; and wherein theturbine section includes a counter-rotating low pressure turbinecomprising an outer rotor directly driving the shaft, the outer rotorincluding a first set of blades, an inner rotor having a second set ofblades interspersed with the first set of blades, the inner rotorconfigured to rotate in an opposite direction about the axis of rotationfrom the outer rotor, a gear system coupling the inner rotor to theshaft and configured to rotate the second set of blades at a fasterspeed than the first set of blades, wherein the gear system is supportedby a mid-turbine frame that is disposed between the high pressureturbine section and the low pressure turbine section; and a firstbearing supporting a fore end of the inner rotor for rotation relativeto the shaft, a second bearing supporting an aft end of the inner rotorfor rotation relative to the shaft, and a third bearing supporting theshaft for rotation relative to the mid-turbine frame.
 4. The gas turbineengine according to claim 3, wherein the first, second, and thirdbearings are axially spaced apart from each other parallel to the axisof rotation with the third bearing being located upstream from the firstbearing.
 5. The gas turbine engine according to claim 3, wherein thefirst and third bearings are roller bearings and the second bearing is aball bearing.